You don't tell the for loop to iterate through any records.

let t := this;
let getrecs := (select UNITS);
let getIndex := index(getrecs, t);
let i := 0;
for i from 1 to 11 do
    t.(Zahl := i);
    index(getrecs, i);
    i := i + 1
end

Line 1: you gather the current record into variable t.

Line 2: you gather all of the records in the UNITS table into variable getrecs.

Line 3: you get the index value of the current record in the array in the variable getrecs.

Line 4: you set the variable i to equal 0. FYI, not something you need to do in Ninox.

Line 5: you start your for loop but have it loop through numbers 1 - 10. You may want to consider:

for i from getrecs do
    i.(Zahl := index(getrecs, i) + 1);
end

Now the for loop says for each item in getrecs do the following.